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Sunday, April 24, 2016

Always study the standard libraries (2)

Here we have another example of the importance of the standard libraries. It all started with this question here, posted at StackOverflow:

The code given by the OP could be fixed, but it had some problems. The main one is the fact it is too particular. It had lines like

while row.length >= 13

This makes it specific to solve the problem proposed for a single value (13). This is something we must avoid at all costs. Never write code tar particular. It is not worth the effort. In a purely theoretical problem like this one can't see this easily. But if you do that in a system designed for production you'll regret it the first time you need to perform a maintenance.

Then I suggested this code, more generic but basically with the same structure:

sequence = "73167176531330624919225119674426574742355349194934

96983520312774506326239578318016984801869478851843

85861560789112949495459501737958331952853208805511

12540698747158523863050715693290963295227443043557

66896648950445244523161731856403098711121722383113

62229893423380308135336276614282806444486645238749

30358907296290491560440772390713810515859307960866

70172427121883998797908792274921901699720888093776

65727333001053367881220235421809751254540594752243

52584907711670556013604839586446706324415722155397

53697817977846174064955149290862569321978468622482

83972241375657056057490261407972968652414535100474

82166370484403199890008895243450658541227588666881

16427171479924442928230863465674813919123162824586

17866458359124566529476545682848912883142607690042

24219022671055626321111109370544217506941658960408

07198403850962455444362981230987879927244284909188

84580156166097919133875499200524063689912560717606

05886116467109405077541002256983155200055935729725

71636269561882670428252483600823257530420752963450"

## This will remove all \n

seq = sequence.tr("\n","")

def multiply_arr(a)

prod = 1

a.each do |f|

prod = prod * f.to_i

end

prod

end

def max_product(s,n)

max = -1

lim = s.length - n

(0..lim).each do |pos|

ns = s.slice(pos,n)

arr = ns.each_char.to_a

prod = multiply_arr(arr)

max = (prod > max) ? prod : max

end

max

end

Good code. Works fine. It is more generic than the code given by the OP. But...

The user Jörg W. Mittag proposed the following code:

def max_product_subsequence(sequence, subsequence_length)

sequence.

gsub(/\s+/, '').

each_char.

map(&:to_i).

each_cons(subsequence_length).

map {|subseq| subseq.reduce(:*) }.

max

end

As you may see, this code does the same mine does, but is is much shorter and much more efficient. I benchmarked it and the execution time is much better.

This is a great example of the importance of the standard libraries. Core Ruby provides wonderful functionalities, if you know it well and apply whenever it is possible.

Ah... before I forget... People say good programmers are always lazy. They need to be lazy in order to try to always find the easiest way to do things. But this does not include being lazy to rewrite code.

Never believe your code is good. Always try to find another way to rewrite it.